我寻找一个好的方法来复制文件(二进制或文本)。我写了几个样本，每个人都能工作。但我想听听经验丰富的程序员的意见。
我错过了好的例子，并寻找一种与c++一起工作的方法。
ANSI-C-WAY
#include <iostream>
#include <cstdio> // fopen, fclose, fread, fwrite, BUFSIZ
#include <ctime>
using namespace std;
int main() {
clock_t start, end;
start = clock();
// BUFSIZE default is 8192 bytes
// BUFSIZE of 1 means one chareter at time
// good values should fit to blocksize, like 1024 or 4096
// higher values reduce number of system calls
// size_t BUFFER_SIZE = 4096;
char buf[BUFSIZ];
size_t size;
FILE* source = fopen("from.ogv", "rb");
FILE* dest = fopen("to.ogv", "wb");
// clean and more secure
// feof(FILE* stream) returns non-zero if the end of file indicator for stream is set
while (size = fread(buf, 1, BUFSIZ, source)) {
fwrite(buf, 1, size, dest);
}
fclose(source);
fclose(dest);
end = clock();
cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
cout << "CPU-TIME START " << start << "\n";
cout << "CPU-TIME END " << end << "\n";
cout << "CPU-TIME END - START " << end - start << "\n";
cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";
return 0;
}
POSIX-WAY (K&R在“C编程语言”中使用这个，更低级)
#include <iostream>
#include <fcntl.h> // open
#include <unistd.h> // read, write, close
#include <cstdio> // BUFSIZ
#include <ctime>
using namespace std;
int main() {
clock_t start, end;
start = clock();
// BUFSIZE defaults to 8192
// BUFSIZE of 1 means one chareter at time
// good values should fit to blocksize, like 1024 or 4096
// higher values reduce number of system calls
// size_t BUFFER_SIZE = 4096;
char buf[BUFSIZ];
size_t size;
int source = open("from.ogv", O_RDONLY, 0);
int dest = open("to.ogv", O_WRONLY | O_CREAT /*| O_TRUNC/**/, 0644);
while ((size = read(source, buf, BUFSIZ)) > 0) {
write(dest, buf, size);
}
close(source);
close(dest);
end = clock();
cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
cout << "CPU-TIME START " << start << "\n";
cout << "CPU-TIME END " << end << "\n";
cout << "CPU-TIME END - START " << end - start << "\n";
cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";
return 0;
}
KISS-C + + -Streambuffer-WAY
#include <iostream>
#include <fstream>
#include <ctime>
using namespace std;
int main() {
clock_t start, end;
start = clock();
ifstream source("from.ogv", ios::binary);
ofstream dest("to.ogv", ios::binary);
dest << source.rdbuf();
source.close();
dest.close();
end = clock();
cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
cout << "CPU-TIME START " << start << "\n";
cout << "CPU-TIME END " << end << "\n";
cout << "CPU-TIME END - START " << end - start << "\n";
cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";
return 0;
}
COPY-ALGORITHM-C + +收费方法
#include <iostream>
#include <fstream>
#include <ctime>
#include <algorithm>
#include <iterator>
using namespace std;
int main() {
clock_t start, end;
start = clock();
ifstream source("from.ogv", ios::binary);
ofstream dest("to.ogv", ios::binary);
istreambuf_iterator<char> begin_source(source);
istreambuf_iterator<char> end_source;
ostreambuf_iterator<char> begin_dest(dest);
copy(begin_source, end_source, begin_dest);
source.close();
dest.close();
end = clock();
cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
cout << "CPU-TIME START " << start << "\n";
cout << "CPU-TIME END " << end << "\n";
cout << "CPU-TIME END - START " << end - start << "\n";
cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";
return 0;
}
OWN-BUFFER-C + +收费方法
#include <iostream>
#include <fstream>
#include <ctime>
using namespace std;
int main() {
clock_t start, end;
start = clock();
ifstream source("from.ogv", ios::binary);
ofstream dest("to.ogv", ios::binary);
// file size
source.seekg(0, ios::end);
ifstream::pos_type size = source.tellg();
source.seekg(0);
// allocate memory for buffer
char* buffer = new char[size];
// copy file
source.read(buffer, size);
dest.write(buffer, size);
// clean up
delete[] buffer;
source.close();
dest.close();
end = clock();
cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
cout << "CPU-TIME START " << start << "\n";
cout << "CPU-TIME END " << end << "\n";
cout << "CPU-TIME END - START " << end - start << "\n";
cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";
return 0;
}
LINUX-WAY //要求内核>= 2.6.33
#include <iostream>
#include <sys/sendfile.h> // sendfile
#include <fcntl.h> // open
#include <unistd.h> // close
#include <sys/stat.h> // fstat
#include <sys/types.h> // fstat
#include <ctime>
using namespace std;
int main() {
clock_t start, end;
start = clock();
int source = open("from.ogv", O_RDONLY, 0);
int dest = open("to.ogv", O_WRONLY | O_CREAT /*| O_TRUNC/**/, 0644);
// struct required, rationale: function stat() exists also
struct stat stat_source;
fstat(source, &stat_source);
sendfile(dest, source, 0, stat_source.st_size);
close(source);
close(dest);
end = clock();
cout << "CLOCKS_PER_SEC " << CLOCKS_PER_SEC << "\n";
cout << "CPU-TIME START " << start << "\n";
cout << "CPU-TIME END " << end << "\n";
cout << "CPU-TIME END - START " << end - start << "\n";
cout << "TIME(SEC) " << static_cast<double>(end - start) / CLOCKS_PER_SEC << "\n";
return 0;
}
环境
GNU / LINUX (Archlinux) 内核3.3 GLIBC-2.15, libstdc++ 4.7 (GCC- libs)， GCC 4.7, Coreutils 8.16 使用RUNLEVEL 3(多用户，网络，终端，无GUI) INTEL SSD-Postville 80gb，最多可填充50% 复制一个270 MB的OGG-VIDEO-FILE
复制步骤
1. $ rm from.ogg
2. $ reboot # kernel and filesystem buffers are in regular
3. $ (time ./program) &>> report.txt # executes program, redirects output of program and append to file
4. $ sha256sum *.ogv # checksum
5. $ rm to.ogg # remove copy, but no sync, kernel and fileystem buffers are used
6. $ (time ./program) &>> report.txt # executes program, redirects output of program and append to file
结果(CPU时间使用)
Program Description UNBUFFERED|BUFFERED
ANSI C (fread/frwite) 490,000|260,000
POSIX (K&R, read/write) 450,000|230,000
FSTREAM (KISS, Streambuffer) 500,000|270,000
FSTREAM (Algorithm, copy) 500,000|270,000
FSTREAM (OWN-BUFFER) 500,000|340,000
SENDFILE (native LINUX, sendfile) 410,000|200,000
文件大小不会改变。 Sha256sum打印相同的结果。 视频文件仍然可以播放。
问题
你喜欢什么方法? 你知道更好的解决方案吗? 你在我的代码中发现什么错误了吗? 你知道回避解决方案的理由吗? FSTREAM (KISS, Streambuffer) 我真的很喜欢这个，因为它真的很短很简单。据我所知，操作符<<对于rdbuf()是重载的，不转换任何东西。正确吗?
谢谢
更新1 我以这种方式更改了所有示例中的源代码，文件描述符的打开和关闭都包含在clock()的测量中。源代码中没有其他重大更改。结果没有改变!我也花时间仔细检查我的结果。
更新2 while循环的条件不再调用feof()，而是我将fread()移动到条件中。看起来，代码运行速度快了10000个时钟。
度量改变了:以前的结果总是被缓冲，因为我重复了旧的命令行rm to。为每个程序执行几次Ogv && sync && time ./program。现在我为每个程序重新启动系统。未缓冲的结果是新的，并不令人惊讶。没有缓冲的结果并没有什么变化。
如果我不删除旧的副本，程序的反应不同。使用POSIX和SENDFILE覆盖现有的缓冲文件更快，所有其他程序都较慢。也许截断或创建选项对这种行为有影响。但是用相同的副本覆盖现有文件并不是一个真实的用例。
使用cp执行拷贝需要0.44秒未缓冲，0.30秒缓冲。所以cp比POSIX样本要慢一点。对我来说还好。
也许我还从boost::filesystem中添加了mmap()和copy_file()的示例和结果。
更新3 我也把它放在博客页面上，并做了一些扩展。包括splice()，这是一个来自Linux内核的低级函数。也许接下来会有更多的Java示例。 http://www.ttyhoney.com/blog/?page_id=69
